Isotropic charge screening of the anisotropic black phosphorus revealed by potassium adatoms

Recent angle-resolved photoemission spectroscopy and transport experiments have shown that at the macroscopic level the electronic structure of black phosphorus can be greatly modified by potassium adatoms. Understanding the effects of individual potassium adatoms at the microscopic level is of great importance. To this end, we use scanning tunneling microscopy to study black phosphorus with potassium adatoms. We find that the potassium atoms are almost fully ionized even at 4.5 K. The Fermi level of black phosphorus is shifted due to electron doping while the band gap shows no significant change. Due to the puckered anisotropic lattice structure, we find that potassium ions can easily migrate along the zigzag direction but not along the armchair direction. Using tip as a moving gate, we can control the ionization of potassium ions. With this technique, we probe the screening effect of charged impurities in black phosphorus at the atomic scale. Remarkably, we find that it shows in-plane isotropic screening behavior even though the underlying electronic dispersion and lattice structure has distinct anisotropy. We also construct two potassium-adatom clusters as a coupled system to study the interaction of them. These results reveal the rich anisotropic physics in black phosphorus.